Band-gap voltage references are used as voltage references in electronic systems. The energy band-gap of Silicon is on the order of 1.2V, and is independent from temperature and power-supply variations. Bipolar transistors have a negative temperature drift with respect to their base-emitter voltage (Vbe decreases as operating temperature increases on the order of −2 mV/deg C). However, the thermal voltage of a bipolar transistor has a positive temperature drift (Vt=kT/q, thus Vt increases as temperature increases). The positive temperature drift in the thermal voltage (Vt) may be arranged to compensate the negative temperature drift in the bipolar transistor's base-emitter voltage. Band-gap reference circuits use the inherent characteristics of bipolar transistors to compensate for temperature effects and provide a stable operating voltage over various power-supply and temperature ranges.
An example band-gap reference circuit is illustrated in FIG. 5. As shown in the figure, two bipolar transistors (Q51, Q52) are arranged with a common base. Two resistors (R51, R52) are series connected between the emitter of the first bipolar transistor (Q51) and the reference output. Another resistor (R53) is connected between the emitter of the second bipolar transistor and the reference output. An error amplifier (EAMP) is used to adjust the voltage of the reference output (VREF) via transistor MLDO. At steady-state, the voltage at the common point of resistors R51 and R52 is the same as the voltage at the emitter of the second bipolar transistor (Q55). The two bipolar transistors (Q51, Q52) are arranged to provide a ten-to-one (10:1) current density difference with respect to one another. The ten-to-one current density results in a 60 mV difference between the base-emitter voltages of two bipolar transistors(ΔVbe=Vt*1n(I1/I2)=26 mV*1n(10)=60 mV, at room temperature). The 60 mV difference appears across the first resistor (R51). The voltage between the drain of transistor MLDO and ground provides a voltage reference (VREF) that is given asVREF=Vbe+X*Vt, where X is a constant that is used to scale the temperature correction factor. The temperature correction factor (X) is adjusted by the ratio of the resistors. Typical temperature corrected reference voltages of 1.25V are achieved by this configuration.